Applicability Statement 2 (or “AS2”) is a specification for sending messages securely over the Internet. In this specification, application-level messages (hereinafter simply “messages”) are typical sent using the HyperText Transfer Protocol (HTTP) or HTTP Secure (HTTPS) protocol. Following AS2, each original message normal requests an acknowledgement in the form of a Message Disposition acknowledgement (or “MDN”), which is a separate application-level message dispatched in the body of an HTTP or HTTPS message or perhaps even by e-mail.
The MDN includes a receipt for the original message. That receipt may be signed or unsigned by the recipient of the original message and the sender of the acknowledgement on the original message. The term Non-Repudiation of Receipt (NRR) is often used in combination with such receipts, and refers to a legal event that occurs only when the sender of the original message has verified a signed receipt coming back from recipient of the original message by confirming that the receipt was properly signed by the recipient, and by confirming that the signed receipt correlates to the original message.
Although not part of the AS2 specification, at least at present, the publish/subscribe paradigm also has some relevance for the principles described herein, and thus will be summarized. Publish/subscribe (or pub/sub) is an asynchronous messaging paradigm where senders (publishers) of messages are not programmed to send their messages to specific receivers (subscribers). Rather, published messages are characterized into classes, without knowledge of what (if any) subscribers there may be on each of those classes. Subscribers express interest in one or more classes, and only receive messages that are of interest, without knowledge of what (if any) publishers there are. This decoupling of publishers and subscribers can allow for greater scalability and a more dynamic network topology, and is implemented in a large number of messaging platforms.